1. Field of the Invention
The present invention relates to apparatus and methodology for electronically determining the qualitative and quantitative physical state, in real time, of either inorganic and organic materials by monitoring their electrical properties for data acquisition, manipulation, analysis and system control.
2. Background of the Invention
Radio frequency (RF) generators used in tissue ablation medical procedures provide RF energy between one or more electrodes supported on an ablation catheter and a ground electrode applied to the patient; alternatively, the catheter may include one or more rings or other electrode(s) that serve in the manner of a ground electrode. The temperature of the catheter obtained from a thermocouple or thermistor embedded in the electrode tip of the catheter is usually used to control the delivery of ablation energy. While such a thermocouple/thermistor measurement may be sufficiently accurate to reflect the temperature of the catheter tip electrode, it is inherently inaccurate and imprecise in determining the temperature of the tissue during ablation (Hindricks, et. al., xe2x80x9cRadiofrequency coagulation of ventricular myocardium: Improved prediction of lesion size by monitoring catheter tip temperaturexe2x80x9d, Eur Heart Journal 1989; 10:972-984; Langberg et. al., xe2x80x9cTemperature monitoring during radiofrequency catheter ablation of accessory pathwaysxe2x80x9d, Circulation 1992;86:1469-1474; Haines et. al., xe2x80x9cObservation on electrode-tissue interface temperature and effect on electrical impedance during radiofrequency ablation of ventricular myocardiumxe2x80x9d, Circulation 1990;82:1034-1038; Blouin, et. al, xe2x80x9cAssessment of effects of radiofrequency energy field and thermistor location in an electrode catheter on the accuracy of temperature measurementxe2x80x9d, PACE 1991; Part I 14:807-813.). Because of the thermocouple""s (thermistor""s) inability to accurately reflect tissue temperature, there is a propensity to overheat the ablation site, which could lead to four potentially injurious conditions (He, et. al., xe2x80x9cTemperature monitoring during RF energy application without the use of the thermistors or thermocouplesxe2x80x9d, (abstract) PACE 1996; 19:626; He, et. al., xe2x80x9cIn vivo experiments of radiofrequency (RF) energy application using bio-battery-induced temperature monitoringxe2x80x9d, (abstract) J. Am Coll Cardiol 1997; 29:32A). First, the delivery of RF energy from the catheter tip may become ineffective due to blood coagulation, and furthermore, the coagulum can be dislodged into the blood stream potentially causing a stroke due to occlusion of downstream blood vessels in critical organs. Second, overheated tissue at the ablation site may xe2x80x9cstick toxe2x80x9d the catheter tip and result in tearing of the tissue upon removal of the catheter. Third, inadequate tissue temperature control can result in unnecessary injury to the heart, including perforation. Fourth, extreme heating of the tissue can cause one or more micro-explosions, which micro-explosions are undesirable since they may displace a piece of tissue into the blood stream and possibly cause a stroke. Because of these potentially dangerous conditions, an apparatus for accurately determining, on a real time basis, the state of tissue during ablation would be of great advantage in performing medical electrophysiological (EP) procedures. Furthermore, if prevention of micro-explosions is not possible for any reason, perhaps due to tissue abnormalities, it would also be advantageous if the apparatus would identify the occurrence of a micro-explosion(s) so that the patient can be closely monitored for adverse reaction during and after the procedure.
Another factor that complicates a cardiac EP procedure in a dynamic beating heart with blood flow is the quality of the electrode-tissue contact. The main aim of an EP procedure is to damage a selected site on cardiac tissue to interrupt errant electrical pulses that cause arrhythmias. To ensure a successful EP procedure, consistent and reliable lesion creation is needed. However, in order to create lesions in a consistent and reliable manner, it is necessary to have good physical contact between the active catheter electrode and the tissue surface. Prior art generators (such as ultrasound, RF, and cryogenic generators) include no device or mechanism that provides adequate information on the quality of electrode-tissue contact. Methods used by cardiologists to determine contact include monitoring the ECG injury current signal before and during energy delivery, monitoring tissue impedance and electrode temperature during energy delivery. (Strickberger, et. al., xe2x80x9cRelation between impedance and endocardial contact during radiofrequency catheter ablationxe2x80x9d, American Heart Journal 1994; 128:226-229; Avitall, et. al., xe2x80x9cThe effects of electrode-tissue contact on radiofrequency lesion generationxe2x80x9d, PACE 1997; 20:2899-2910). The prior art technology for determining the quality of the electrode-tissue contact is inefficient and ineffective for various reasons. First, because the contact impedance can only be faintly determined during the delivery of energy, this approach unnecessarily lengthens the duration of the procedure. To help illustrate this point, consider the following presently practised methodology. A physician first has to maneuver the catheter with the aid of a fluoroscope to the site on the tissue to be ablated. The physician would then have to deliver a small dose of RF energy so that the change in temperature or impedance signals can be monitored for approximately 30 to 40 seconds. If the monitored signals do not indicate sufficient contact, the whole process would have to be repeated until sufficient contact was achieved. This repetitive process also subjects the patient to unnecessary exposure to radiation from the fluoroscope. Second, the difference between poor contact (i.e. catheter electrode lightly touching the tissue) and good contact (i.e. catheter electrode firmly in contact with tissue) is of insufficient resolution due to inadequate sensitivity of the impedance measurements in prior art RF generators. One prior art RF generator typically provides an approximate initial impedance of 113xc2x116xcexa9 (mean xc2x1SD) for poor contact and 139xc2x124xcexa9 for good contact (Strickberger, et. al., xe2x80x9cRelation between impedance and endocardial contact during radiofrequency catheter ablationxe2x80x9d, American Heart Journal 1994; 128:226-229). The approximate changes in impedance during a short ablation with another prior art generator (40 second application of 20 W of RF energy) are 14xc2x110xcexa9 for poor contact and 20xc2x12xcexa9 for good contact (Avitall, et. al., xe2x80x9cThe effects of electrode-tissue contact on radio frequency lesion generationxe2x80x9d, PACE 1997; 20:2899-2910). These data show that the difference in the initial impedance (measured during short duration low energy delivery) that is used to estimate good contact is only 26xcexa9 and the large standard error makes this measurement uncertain since there is substantial overlap in the values measured. In the second example the difference in the change of impedance (during ablation) is only 6xcexa9. Third, the technique using the change in temperature for estimating contact pressure can not be used effectively with an irrigated or chilled catheter since the electrode of the catheter is intentionally not responsive to increases in tissue temperature, unlike a non-cooled catheter. Due to the above reasons, cardiologists find these existing approaches of determining contact quality cumbersome and inefficient, and usually skip the process. Thus, an apparatus capable of effectively monitoring and providing accurate and sensitive tissue impedance measurement before, during, and after an EP procedure as well as quantifying with sufficient resolution the contact quality for discriminating between no, poor, fair, and good electrode-tissue contact would be of great utility and advantage in performing an EP procedure.
Catheter ablation treatment of atrial fibrillation often requires the formation of linear lesions and/or circumferential lesions and treatment of ventricular tachycardia typically requires formation of deep lesions. If tissue lesion formation below an electrode could be accurately monitored, it could improve the ability to produce a continuous line of lesions required for some ablation treatments of atrial fibrillation. Additionally, accurate monitoring could provide an indication of successful lesion formation for treatment of ventricular tachycardia. Thus, an apparatus and a method for monitoring lesion formation, size and depth, features not provided by prior art generators, would be of great advantage.
A similar need for real-time monitoring of inorganic materials is present in many industrial applications. RF welding, for example, typically requires two plastics or metals to be heated and permanently joined. However, not all materials can be electrically heated efficiently or within practical limits. For example, no amount of electrical energy can weld two purely reactive plastics and attempting to weld purely reactive materials could result in expensive damage to the welding system. Therefore, an apparatus that can quantify and qualify the weldability of inorganic materials in real time would be of immense utility.
Implementing a system efficiently requires electrical matching for optimal energy consumption. For example, an unmatched RF heating and welding system requires more available power than a matched RF heating and welding system due to significant reflection of applied energy. This implies that implementation of a design using an unmatched RF welding system would not only be more expensive, due to the expensive higher power components as well as the higher energy consumption, but also more complex since heat compensation and management would become important. Electrical mismatch occurs for a variety of reasons, including changes in the properties of a material due to its chemical formulation, storage environment, aging, and during heating. An apparatus that monitors the electrical properties of a material in real time prior to and during heating or welding would allow an operator to manually, or allow the apparatus to automatically, tune the matching circuit to achieve and sustain optimal matching for increased energy consumption efficiency and for protecting the system from damage.
The present invention analyzes and/or treats organic or inorganic material and produces one or more outputs and may include one or more subsystems. The function of a first subsystem is to measure the electrical properties of the material and present these properties as inputs to a second subsystem. The electrical properties may be measured at an appropriate frequency or frequencies (for the purpose of demonstrating the implementation and application of the invention, a 50 KHz monitoring signal for an RF ablation system has been presented). A change in the current of the monitoring signal is reflective of a change in the state and properties of the material. The output signals of the first subsystem include measurement of the magnitude of the resistive component, the magnitude of the reactive component, and the impedance magnitude, all being measured across the material being analyzed or treated. The function of the second subsystem is to process these output signals, either individually or in combination, in digital or analog fashion, and to provide a quantitative measure of the property or state of the material for analysis and/or control purposes. The output of the second subsystem provides quantitative and/or qualitative signals and control signals. The quantitative and/or qualitative signals are signals for quantifying and/or qualifying events such as contact quality, lesion formation, etc., while the control signals are used for control purposes.
The present invention can be used for monitoring and controlling treatment of tissue during a cardiac electrophysiology ablation procedure, during procedures for heating or freezing of cancerous tumors, during procedures for management of spinal pain, and for other medical therapeutic procedures by providing accurate real time measurements of the impedance and the resistive and reactive electrical components of the tissue by exploiting real time electrical parameters as described herein. Alternatively, the present invention can be used to characterize plastic for quality control and to measure the quality of plastic forming and welding and in other medical and industrial applications.
It is therefore a primary object of the present invention to provide accurate and sensitive measurements of the resistive component, the reactive component, and the impedance of either organic or inorganic material in real time before, during, and after application of energy.
Another object of the present invention is to provide apparatus for verifying, assuring, and qualitatively and quantitatively assessing real time electrode-tissue contact to present information during an EP procedure before, during, and after the application of energy.
Yet another object of the present invention is to provide apparatus for monitoring and producing real time information that can be correlated with as well as qualifying and quantifying lesion volume and depth and also estimating tissue temperature.
Still another object of the present invention is to provide apparatus to safely control lesion depth and volume and tissue temperature to user specified values during an EP procedure.
Still another object of the present invention is to provide apparatus to detect tissue micro-explosions or other events such as slippage of a catheter during an EP ablation procedure.
Still another object of the present invention is to provide apparatus for producing signals reflective of differentiation between healthy and damaged tissue (such as infarct tissue), between different tissue types and textures, and discriminating between healthy tissue and adjacent unhealthy tissue (such as a tumor).
A yet further object of the present invention is to provide apparatus for producing a signal providing measurement of a material""s electrical properties and any variation caused by the storage environment or age and differentiation between similar materials of different chemical formulation or age.
A yet further object of the present invention is to provide apparatus for producing a signal for preventing arcing during welding by identifying the existence of an air gap caused by missing, imperfect, or defective material and identifying the existence of inclusions or the presence of foreign matter on the surface of the material.
A yet further object of the present invention is to provide apparatus for identifying and quantifying electrically modifiable materials (such as RF responsive plastics) and accurately measuring the electrical properties of the materials for manual or automatic adjustment of a matching circuit to increase the energy efficiency of the apparatus.
A yet further object of the present invention is to provide apparatus for generating a signal useful for estimating and controlling the temperature of a material and/or change in temperature during a heating and welding process.
A still further object of the present invention is to provide apparatus for generating a signal determinative of a weld formed and the quality of the weld.
A still yet further object of the present invention is to provide a method for monitoring and controlling treatment of tissue during an ablation procedure.
A still further object of the present invention is to provide a method for distinguishing between healthy and abnormal or diseased tissue.
A still further object of the present invention is to provide a method for determining electrical properties of organic and inorganic materials.
A still further object of the present invention is to provide a method for determining the quality of the contact between an energy transmitting electrode and an underlying material prior to transmission of energy to modify the material.
A still further object of the present invention is to provide a method for generating control signals to permit control of the power and/or duration of energy transmission to an organic or inorganic material.
These and other objects of the present invention will become apparent to those skilled in the art as the description thereof proceeds.